The accurate measurement of the motion and deformation of a given shape has many applications, such as in the fields of machine vision, industrial automation, scientific biomechanics research, medical treatment, and 3D animation among others. The shapes whose deformation, movement, and motion are measured include objects of interest, manufactured parts, body parts, etc. The measurements can be used for object differentiation, where the object differentiation is based on material, size, shape, and cost, among many other parameters. When the shape being measured is a portion of a body such as the human body, then shape measurement has further applications in industries such as sports, healthcare, and 3D animation for entertainment and gaming. Accurate measurement can be used to obtain data related to personal medical information and to design medical treatments. Proper medical treatments are essential for comfort, safety, and therapeutic outcomes.
In a clinical setting, accurate and precise human body measurements are difficult to obtain. To start with, consider a relatively simple, static, volumetric body part measurement, such as measuring the volume of fluid buildup in a limb caused by lymphedema. This is typically a manual process where a tape measure is often used by a clinical professional to make body measurements. First the limb is marked along a longitudinal axis using the tape measure and a marking pen. An appropriate gradation, say every 1 cm, is marked. Next, a transverse circumference is measured at every gradation and recorded. The transverse circumferential measurements are repeated along the desired length of the limb. At a subsequent clinical visit, say one week or one month later, the measurements are taken again. Total limb volume V can be approximated by assuming a step-wise linear series of cylindrical disks. The volume V can be expressed as the area A of each transverse cross-section (where A=C2/4π, and where C is the measured circumference) times the height h of each gradation, and then summing all of the cylindrical disk volumes into the total volume. In this way, lymphedema progression and/or treatment effectiveness can be monitored.
Unfortunately, even though this is a relatively simple example involving a static measurement of a non-moving body part, the typical clinical approach is fraught with inconsistencies and opportunities for human error. A different person may be making the measurements. Inconsistent pressure may be applied when measuring the circumference. The tip of the marking pen can be several mm wide. Subtle limb shape changes, whether related to lymphedema or not, may greatly affect the accuracy of the estimated volumetric model calculation. Many such difficulties exist for making even this relatively simple static, body part measurement.
While making static body part measurements is very difficult, it is even more difficult to measure moving body parts, such as a joint. Body part joint movement is three-dimensional, and the movement happens in real-time, that is, non-static. By necessity, the body part joint is moving when a measurement needs to be taken. Body part joint measurements can involve different deformations along multiple axes. Multiple measurements of a repetitive motion may be required. Measurements may need to be made while the body part is under a load condition or under nominal conditions. All of these variables present an additional layer of variation that makes measurement difficult. Added to all that complexity is the fact that body part joints are connected to other body part joints, which further complicates measurement and analysis of shape motion and deformation. Accordingly, a great need exists to be able to accurately measure and analyze body part motion.